multiplexing lcd displays brands

Driving a segment LCD using multiplexing reduces the number of pins required to turn on or off the segments of a display. This application note discusses what a segment display is and the driving method in detail. The SDAF102NCRN01, a 3V, 7-segment custom LCD is used as an example.

Segment LCDs are constructed using two pieces of Indium Tin Oxide (ITO) glass with a twisted nematic fluid sandwiched in between. The majority of these displays are custom-made. Typical applications include measuring acidity levels in swimming pools, gases, or temperature. There are two types: static and multiplexed. A static display is a segment display with one pin for every one segment, whereas a multiplexed LCD has grouped segments, reducing the number of pins.

So, what is a segment? A segment is any line, dot or symbol that can be turned on and off independently. The figure below shows an example of a custom display developed by Focus LCDs.

The number of segments is dependent on what will be displayed. The most popular are seven-segment displays. In Figure 1, the digits: “0”, “8”, “4”, “7” are all seven segments. Each segment can be independently turn on or off to show a letters or number. However, the range of letters are limited. While fourteen segments have the ability to display any number and more letters.

Icons such as symbols for battery, signal strength, plus/minus and bar graphs are also considered segments. Finally, segments can also be “permanent”. This means it is always on even with no power. This is accomplished by burning the segment onto the glass. The text: “FOCUSLCDS.COM” in Figure 1 is an example.

The multiplexing technique aims to reduce the number of pins that are necessary for driving the segments of the display. This results in a simplified LCD module. In this setup, each segment control line can be connected to as many segments as there are backplanes, provided that each of the connected segments are tied to separate backplanes. This method "multiplexes" each of the segment control lines and minimizes the number of pins. The advantage of this is increased display density and reliability at the expense of complicated drive circuitry.

Segments are turned on or off using an AC voltage with no DC component (Figure 4). This means that the average voltage of the AC waveform should be equal to zero. Having a DC bias will reduce the life of the display. Also, there must be always an AC voltage on all the segments of the LCD. A sign that the LCD is degrading is when there is a loss of alignment on the edge of the characters, resulting in a distorted visual appearance.

Crosstalk, or “ghosting”, occurs when an LCD is overdriven by a combination of frequency and voltage. This appears as a partial turning on or off of a segment. To prevent inadvertent turning on or off of the segments, unused segments must be connected to its backplane (COM) pins.

One major disadvantage of multiplexed drivers is reduced contrast due to a lower duty cycle. In this case, a segment is on 25% of the time, while in static-driven displays have sharper contrast from being on 100% of the time. However, to the human eye this decrease in contrast is not noticeable.

Buyers and others who are developing systems that incorporate FocusLCDs products (collectively, “Designers”) understand and agree that Designers remain responsible for using their independent analysis, evaluation and judgment in designing their applications and that Designers have full and exclusive responsibility to assure the safety of Designers" applications and compliance of their applications (and of all FocusLCDs products used in or for Designers’ applications) with all applicable regulations, laws and other applicable requirements.

Designer agrees that prior to using or distributing any applications that include FocusLCDs products, Designer will thoroughly test such applications and the functionality of such FocusLCDs products as used in such applications.

1TN (Twisted Nematic) is a popular LCD technology that does not require current flow for TN cells to work and uses lower operating voltages, making them suitable for portable applications.

We receive many calls regarding direct drives and multiplex drives for custom segment LCD displays. The Segment LCD is a common type of custom display module; in fact, it is the lowest cost custom LCD glass option available. One reason for the low cost is that it does not contain a controller/driver chip. Therefore, instead of having a parallel / IC2 / SPI interface options, you now have two options: Direct (or static) drives and multiplex drives.

There are two drive types available for a custom segmented LCD module. Direct drives or static and multiplex drives. There is no difference in tooling cost between the two. Before we explain the difference between direct and multiplex drives, let’s talk about segments.

Figure one below is a custom segmented LCD module. The number ‘8’ is made up of seven (7) independent segments and a decimal point. Each one of these segments can be turned on and off individually. This allows the user to create any number and many letters such as ‘E’, ‘F’, ‘L’ and others.

Direct drive custom segmented LCDs contain one pin for every segment plus supporting pins such as power, ground and COM’s. Figure one shows eight segments, this would require a total of eight pins for the segments, plus supporting pins.

Figure two shows an LCD with eight number ‘8’s. This equates to 8 numbers multiplied by 7 segments per number, which equals 56 segments. If you were to use direct drive on this display, you would need 56 pins plus supporting pins. The higher quantity of pins not only increases the size of the display , since you will need a larger display to hold all the leads, but it also increases the unit cost and the amount of time to install the LCD on a printed circuit board (PCB)

The most common type of multiplex LCD display used on a custom segment LCD is called a 4:1 mux. The means that there is one pin for every four segments. The advantage of this configuration is that it reduces the number of pins by 75%. Add up all the segments and divide by four and you arrive at the number of pins necessary, plus supporting pins.

One down side to a multiplex custom segmented LCD is the reduced LCD contrast. On a direct drive each segment is on 100% of the time, in a 4:1 mux configuration, each segment is on 25% of the time. The reduced sharpness is not very noticeable and difficult for the human eye to notice.

These types of lcd screens don"t have any driver to light up the elements on the panel. You need a driver to run them and make sure the elements are not damaged (you can"t just turn them on and off like a led, they require a somewhat special signal).

There are microcontrollers which have LCD drivers built in along with multiplexing capability and all that, go for example on Digikey and select microcontrollers: http://www.digikey.com/product-search/en/integrated-circuits-ics/embedded-microcontrollers/ Then select LCD at peripherals and you"ll find microcontrollers that have built in lcd drivers

There are also application notes explaining how to drive such plain lcd displays (without driver on the back) from various companies, just go on Microchip"s site or Atmel"s site and look up the application notes and see the explanations.

Basic Operation of an LCD, LCD Direct Drive Techniques , Multiplexing LCD"s , Multiplexed Liquid Crystal Displays etc : http://www.lxdinc.com/application_notes/index

Dave has a video somewhere in which he searches for a microcontroller capable of driving a LCD without built in driver and explains some things, but really I can"t be bothered to search for it in close to 600 videos. Do some work yourself.

Liquid Crystal Displays or more commonly known as LCDs are one of the most common electronic components which help us interact with an equipment or a device. Most personal portable equipment and even gigantic industrial equipment utilize a custom segment display to display data. For many portable consumer electronics, a segment LCD display is one of the biggest contributors to the overall cost of the device, hence designing a custom segment display can drive the cost down while also utilizing the display area in the most optimum manner. These displays have the lowest cost per piece, low power requirements, and a low tooling fee too.

At first thought, designing a custom segment LCD might look like a Herculean task, but trust me that it is easier than it seems. In this article, we have summarised and compared the display types and available technologies which are required to construct a custom segment LCD. We have also provided a flowchart that can act as a step-by-step guide while you design your own custom LCD. We have also provided the process we followed, a require gathering sheet we used for communicating our needs to the manufacturer, and a few other data and the quotation we received from the manufacturer.

LCD Bias– It denotes the number of different voltage levels used in driving the segments, static drives (explained later in this article) only have 2 voltage levels or 2 bias voltage while multiplex drives have multiple voltage levels. For example, 1/3 will have 4 bias voltages.

LCDs utilizes the light modulating properties of liquid crystals which can be observed by using polarizing filters. Polarizing filters are special materials that have their molecules aligned in the same direction. If the light waves passing through polarisers have the same orientation as the filter, then the molecules of lights are absorbed by the filter, hence reducing the intensity of light passing through it, making it visible.

A custom LCD is important for maximizing the efficiency of the display area by adding custom symbols and characters. It also helps in reducing the cost and improving energy efficiency of the product. A higher number of custom symbols and specified placement of numerical and alphanumerical characters make the display more informative and readable for the user. This makes it look better than the plain old boring displays we get in the market. Furthermore, we can specify the viewing angle, contrast, and other specifications which can increase durability or give a better value for money for our intended usage.  A typical Custom Segment display is shown below, we will also show you how to design and fabricate the same further in the article.

The LCD display doesn’t emit any light of its own, therefore it requires an external source of illumination or reflector to be readable in dark environments.

While designing a custom segment LCD display, we have the leverage of choosing a lot of parameters that affect the final product. From the color of the display to the illumination technique and color of illumination as well as the type of input pins. Some important considerations we need to take while designing a custom 7 segment display are - the type of display, i.e. positive or negative, illumination method, driving technique, polarising type, and connection method. All these design criteria are explained below:

Positive and negative displays can be easily distinguished by the colour of the background and characters. Some common differences between the positive and negative displays are:

So, which one should you choose? When the displays are to be used in areas with higher ambient light, we should select positive segment LCD display as it has better visibility than negative segment LCD displays without using a backlight.

As we know that LED displays don’t emit any light, hence to illuminate it and make it visible in a dark environment, we can use different methods of illumination. The most common LCD Illumination methods are compared below:

For displays that need to be used for budget-friendly devices that should be small and rugged, LED lights are preferred for the displays due to the high durability and low cost of operations. For high brightness, CCFL and Incandescent lights can be used.

A polarizer film is the most important component of an LCD display, which makes it possible to display characters by controlling the light. There are 3 types of polarizers that can be used in the LCD display, the properties and difference are given below:

Displays can be categorized into two types, passive displays, and active display, passive displays are simpler to construct as they have 2 connections at each segment, the conductors comprise of an Indium Tin Oxide to create an image, whereas the active displays use thin-film transistors (TFT) arranged in a grid. The name is due to its ability to control each pixel individually.

If your displays have fewer segments, then static LCD drive is preferred as it is easier to control and cheaper to construct, and has a better contrast ratio. But let’s say that if the number of segments in the display are more than 30-40 then a multiplex LCD drive should be preferred as it has multiple common pins, hence reducing the total number of pins required to drive the display.

Choosing a connector type!!! For the prototyping phase or if you need to connect your LCD display on a Microcontroller directly, a pin type connector is the best and most economical option you have. If you need to connect your LCD display in a final product with a high volume of production which also requires to be extremely durable, but at the same time should not take up a lot of space, a Flex type LCD Connector will work best for you

LCDs have limited viewing angles and when seen from an angle they lose contrast and are difficult to be observed.  The viewing angle is defined by the angles perpendicular to the center of the display towards its right, left, up, and down which are denoted by the notations 3:00, 9:00, 12:00, and 6:00 respectively. The viewing angle of LCD can be defined as the angle w.r.t. to the bias angle at which the contrast of segments is legible.

To improve the viewing angle in an LCD, a Bias is incorporated in the design which shifts the nominal viewing angle with an offset. Another technique is to increase the Voltage, it affects the bias angle, making the display crisper when viewed from a direction.

For example, the viewing angle of a TN type TFT LCD is 45-65 degrees. Extra-wide polarising film (EWP) can increase the viewing angle by 10 degrees, using an O film polariser can make the viewing angles 75 degrees but these come at a cost of reduced contrast.

LCD Control chip or LCD driver chips can be mounted on the flex cable, display, or externally on a PCB. The placement of LCD control chip can affect the cost and size of the display. The 2 most common methods of chip placement are-Chip of Board (COB)and Chip on Glass(COG) which are described below:

We planned to design an air quality monitoring system for which we needed a custom segment LCD panel for an air quality monitoring device. Our product needs to display the following data: 2.5-micron and 10-micron particulate matter (PM) suspended in the air; the units should be in parts per million (PPM). CO2 in the air in PPM along with total volatile organic compounds present in the air in parts per billion (PPB). To make the product more usable, we included time in 24-hour format, Temperature in ºC, Battery status, loudspeaker status, Bluetooth status, and Wi-Fi status. And for some personal touch, we also added how good the air quality in the room is by using 3 different smileys.

We realized that it was impossible to provide all these data in a generic LCD available in the market, thus decided to build a custom LCD for our project.

A step-by-step flowchart is shown below to walk you through each and every step of selecting components and getting your custom segment LCD manufactured.

Usually, the displays are mounted at a height of 4.5 feet from the ground, thus the viewing direction was selected to be 12"O clock with an operating frequency of 64Hz. We selected a Transmissive polarizer for the front glass and a reflective polarizer for the rear glass so that the natural light can pass through the front panel and the display can achieve the maximum contrast without the need for backlighting and we opted for the pin type connectors as they are easy for prototyping and are suitable for harsh environment with a lot of vibrations and shocks which best suited our purpose.

We mailed our requirements to multiple LCD manufacturers, (you will find a lot of LCD manufacturers on the Internet). Most LCD manufacturers have competitive pricing, and reply within a week. A sample requirement sheet is shown above which a customer needs to fill to specify all the details to the manufacturer.

This is a sample Custom Segment LCD quotation we got from one of the manufacturers. As you can see, the cost is based on the quantity. Higher the quantity, lower the cost. Apart from the cost per quantity, there is one more component called tooling fees. Tooling fee is a one-time fee charged by the manufacturer. It is for the technical design, support, and customization of the product. Customization of PCB or tooling of LCD can drive the tooling price higher or lower.

A custom segment LCD can help you personalize your product while also saving the overall cost of your product. The whole process will take you around 2-3 months, which will include the designing phase, prototyping phase, and getting your custom segment LCDs delivered to your doorstep. Higher ordering quantity will reduce the cost per piece of each unit, thus driving down the cost of your final product.

The basic LCD cell has no readily discernible optical characteristics and looks transparent under any condition. But if a piece of properly oriented linear polarizer (such as those found in polarized sunglasses) is placed on each glass surface, the basis of the cell"s display properties is provided.

Users often backlight such "transmissive" displays with any convenient source - incandescent, fluorescent, or neon, for example - or with ambient light.

TN liquid crystals work very well in multiplex applications, however, it is important to understand the restraints that multiplexing applies to the display so that they can be properly implemented.

The most important thing to remember in multiplexing displays is that the elements are all interconnected, and there is a cross-talk problem which must be addressed.

In effect, this makes the effective drive voltage on the display a function of "n," or the number of lines it is multiplexing. So the higher the multiplex ration, the poorer the viewing angle and contrast ratio.

If the LCD panel breaks, do not get the liquid crystal in your mouth. In case of contact with your skin or clothes, wash it off immediately with soap and water.

When soldering displays with pins, avoid excessive heat. Keep the soldering temperature between 260°C and 300°C and apply heat for no more than 5 seconds. Never use wave or reflow soldering. Cover the surface of the LCD to avoid flux spatter. Remove flux residues afterward.

Do not use DC voltage to drive the LCD and keep the voltage within the specified limit. Excess voltage will shorten the display"s life. Within limit, the viewing angle can be fine tuned by varying the voltage (especially for STN). Response time will increase as the temperature decreases.

This category contains multiplexed LCDs without integrated display controller. To control such LCDs you need a microcontroller with integrated LCD controller, or you need to generate the required waveforms in software.

The number of backplanes of a multiplexed LCD determines the multiplex factor and how many LCD segments can be controlled via a single I/O line. For example a 4-way mux LCD has 4 backplanes and a single segment pin controls 4 segments of the LCD. To control n segments of a m-way multiplexed LCD, you will need n/m+m signal lines. For example to control a 40 segment 4-way multiplexed LCD, you need 40/4 + 4 = 14 I/O lines.

While adding more and more to my project i found some difficulties. I run out of digital pins due to my LCD and i would like to control 2 7 seg displays to project some numbers from a temperature sensor.

So my question is if it is possible to output 2 different numbers (first_digit and second_digit) on the 7seg displays. I got some ideas from this thread . I think that the schematic is ok though.